Did we forget about the part where the crew apparently dies inside of a freakish plant that eats them? The end of that film is like a horror movie; it's frightening.
Tone isn't really about what is happening, but how it is happening. The tone of the film is much darker than usual; I don't see how anyone can disagree with that. It's all about the execution of the events, not the actual events (though the events themselves are pretty dark creepy, too).
The execution is so incredibly important in establishing the tone of an artwork; you never see Blackbeard's power spit someone out who is inside-out. Sure, it's implied that he may be able to do something like that to a person, but we never see it. And with the kids on drugs, Oda literally had to sugar-coat that by making the drug a candy. If the children were shown cowering in fear while adults held them down and forced needles into their arms, the execution of the event would have been darker, making the tone significantly darker.
This sort of grotesque, violent imagery distinguishes the tone of the film from any other One Piece film. Obviously, the film is far darker than anything else in the show. Also, keep in mind that those mummified, seemingly dead bodies are his crew.